Chalk versus Marl

Cantclimbtom

Active member
Hello geologists, geotechnical and mining surveyors people

Please may I ask a question about chalk and marl, yes... I have tried Googling it, but for some reason (perhaps my ignorantly formed searches) I'm not getting back what I want. I already understand that chalk is composed of the remains of sea creatures/shell/etc and probably contains clays. I also already understand that marl is precipitated carbonate form a great lake or drying inland sea and that it is more sand/silica (less clay).

But if you are out in the field and someone just handed you a lump of chalky white white rock to identify, is there a test you can do like scraping with a knife blade or running on sandpaper etc to distinguish a chalk from a marl, or are there any other clues such as flint inclusions indicates one rather than the other?

Thanks in advance!

Edit: or any other geological clues/indicators like if is underlying chalk or underlying consolidated sand/sandstone etc
 

Brains

Well-known member
In my experience marl is a lime rich clay and is used commercially in the production of low grade ceramics (bricks, tiles, etc).
The Potteries have numerous marl pits resulting from its extraction. Here it is red coloured sticky goo that used to regularly drown errant teenagers, but most are now reclaimed and landscaped. It can be moved without explosives using a digger/ shovels and is low in strength. Unconsolidated is a good word for it
Chalk is a weak and porous rock with much less clay and silt. It is mainly composed of the massed shells or skeletons of carbonate supported plankton. Blobs of chert (flint) may form within the body of the rock after sedimentation by the remobilisation silica from the skeletons of glass sponges and radiolaria. These blobs will form in layers relating to local redox fronts within the sediment and are NOT related to bedding. The rock is lithified rapidly after deposition, and can not easily be dug by hand or machine compared to marl.
As with many things geological, colour is a poor indicator, but detailed composition and structure is more relevant. Having said that, chalk is USUALLY considered a white rock!
Metamorphosed, the marl will form... Marlstone, with the silt / clay altered to a variety of minerals. The chalk will alter to a marble with inclusions of silicate minerals from the impurities.
In the field, marl will look like clay and crumble into fine bits readily when dry and be plastic / mouldable when wet. Chalk will be quite firm and not mouldable when wet, but both will be extremely slippery!
 

Cantclimbtom

Active member
Thanks Brains, I know of darkish grey chalk and light grey marl, for example at the very bottom of the white cliffs of Dover apparently outcrops a band of marl, that looks like chalk (and if you look at channel tunnel geology it's marked as Marl there also).
From the above could I interpret that as: if it is hard and rock-like when both dry and wet (especially when flint nodules) it's definitely chalk, but if it's crumbly dry and also mouldable when damp, then definitely a marl?

I did look at BGS surveys for the area I'm interested in (but being shy and avoiding mentioning quite where) and it's marked as a layer of light chalk with a note of grey chalk below, however this is at depth and survey in region with very few boreholes or seismic surveys and the survey done was specifically interested in sand, gravel, any oil or gas (not there), brick clays etc and the distinction between chalk types and/or marl wouldn't have interested them. It would explain a lot if there was grey chalky marl under the chalk, but maybe it's just chalk. I plan to take look :) and now you've armed me with info to make a much more educated guess

Thanks!
 

Brains

Well-known member
Thanks Brains, I know of darkish grey chalk and light grey marl, for example at the very bottom of the white cliffs of Dover apparently outcrops a band of marl, that looks like chalk (and if you look at channel tunnel geology it's marked as Marl there also).
From the above could I interpret that as: if it is hard and rock-like when both dry and wet (especially when flint nodules) it's definitely chalk, but if it's crumbly dry and also mouldable when damp, then definitely a marl?

I did look at BGS surveys for the area I'm interested in (but being shy and avoiding mentioning quite where) and it's marked as a layer of light chalk with a note of grey chalk below, however this is at depth and survey in region with very few boreholes or seismic surveys and the survey done was specifically interested in sand, gravel, any oil or gas (not there), brick clays etc and the distinction between chalk types and/or marl wouldn't have interested them. It would explain a lot if there was grey chalky marl under the chalk, but maybe it's just chalk. I plan to take look :) and now you've armed me with info to make a much more educated guess

Thanks!
That could be a reasonable field differentiation, assuming those are the only rock types available. Many other clays and silts could satisfy the mouldable criteria, and where a unit is described as "chalk" it may contain sub units, beds, lenses or layers of other rock types in minor amounts. For example the Lower Carboniferous is often described as massive thickly bedded limestones, but will contain ash layers, shales, coals, porcelanous "bands" etc. If you look at Kingsdale or Ingleboro' they are limestone features, but the benches are formed on weaker layers with possible other rock types. There is a limestone quarry in Ingleton that has a thin coal exposed. Can-of-worms stuff is rock botherering :dig:
 

Andy Farrant

Active member
A marl typically refers to a unconslidated or weakly cemented carbonate rich clay. The lowest part of the Chalk succession in the UK is quite clay-rich and used to be called the 'Chalk Marl'; it is now defined as the West Melbury Marly Chalk Formation. The overlying unit, the Zig Zag Chalk Formation is also quite clay rich. The rest of the chalk is much purer but thin (1-2 cm thick) marl seams are present at intervals in the rest of the succession. These thin marl seams are often the loci for karstic conduits (see Cave and Karst Science Vol 48(2).

The channel tunnel was dug for the most part in the West Melbury Marly Chalk Formation as this was the ideal tunnelling medium, so the tunnel follows the geology. Microfossils were used to identify the optimum tunnel level in the site investigation boreholes across the channel.

I suggest you get a copy of the latest version of the Geologists Association guide to the Chalk of Sussex and Kent (GA Guide No. 74 2 volumes) by Rory Mortimore. See https://geologistsassociation.org.uk/guidesales/
Andy
 
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